Method for pressure filtering and device for carrying out same

ABSTRACT

The present invention relates to a method of and apparatus for pressure filtering, the pressure filter (1) used comprising pressure plates (3,4,5,6), which can be pressed against each other. Pressure chambers (8) are formed between the pressure plates and in each one of the pressure chambers two substantially vertical filter cloths are mounted. In order to loosen the filter cakes (32) from the filter cloths (9), means are used to produce sudden acceleration forces, e.g. directly or indirectly acting blow devices (34,36,38) or blow devices or lifting means, cooperating with stop surfaces, which devices and means are designed to drop the guide rails (12) from a lifted position, and/or shear forces, e.g., devices to displace the lower ends of the filter cloths in respectively opposite directions in the planes of the cloths.

The present invention relates to a method and means for pressurefiltering used to carry out said method according to the preamble of

The invention relates particularly to solutions of problems of looseningand discharging the filter cake from such pressure filters. For thispurpose it has already been suggested that the filter cloths besubjected to vibrations. DE-C 3 341 636 and DE-A 3 527 735 describealready known pressure filters, in which the filter cloths are suspendedfrom slewing brackets and springs which are raised on the pressureplates. The pressure chambers are opened up one at a time and the filtercake is loosened by means of a movable vibration device, which directlyinfluences the suspension rod of the respective filter cloths. However,these constructions have a drawback. The discharge is verytime-consuming, since the filter chambers are emptied one at a time.

In order to empty all the filter chambers simultaneously it is suggestedin U.S. Pat. No. 4,900,454, that the suspension rods of the filtercloths be movably suspended in a horizontal direction in thelongitudinal direction of the pressure filter on two flexibly orelastically suspended guide rails, which are provided with vibrationdevices in order to vibrate all of the suspension rods and thecorresponding filter cloths at the same time. The positions of thesuspension rods when the pressure plates are brought apart are obtainedby means of flexible and/or pivotable spacer means fastened to the endsof the suspension rods, which spacer means connect the suspension rodsto each other and to the movable gable plates of the pressure filter.Such a pressure filter allows a very fast loosening of the filter cakesfrom the filter cloths and consequently short filtering cycle times.

This already known pressure filter functions very satisfactorily formost types of materials, with an excellent cake loosening effect.However, as far as certain quite adhering or in other ways difficult toloosen types of materials are concerned vibrations of filter clothsproduced in this already known pressure filter are not adequate, sincethey do not always result in a reliable and complete discharge of thefilter cakes. Consequently, portions of or even entire filter cakes mayremain in one or several of the filter chambers after a terminatedfilter cake discharge. Such remaining filter cake portions areparticularly found in the upper parts of the filter chambers. Thisresults in an inferior filtering capacity, because during the nextfiltering cycle portions of the available filter chamber space alreadyare occupied by the remaining filter cake residues. Such remainingfilter cake residues usually will remain also during the subsequentfiltering cycles and increasingly stick to the walls of the filterchamber, i.e. the filter cloths. Thus, the deterioration of thefiltering capacity of the pressure filter will be permanent.

Said remaining filter cake residues can at least be partly removed byspraying a rinsing fluid onto the filter cloths, at least during certainfiltering cycles. The filter cake residues will then be brought back toa container or the like for a liquid suspension of filter materials. Thefilter cake residues which have been loosened by washing will then forma circulating load on the pressure filter, which worsens the filteringcapacity of the system. Also, large loosened and washed away filter cakeresidues may lead to shutdowns and/or other disturbances in the rinsingliquid circulation system.

Due to the above-mentioned drawbacks of already known pressure filtersthe purpose of the present invention is to suggest a method ofefficiently and completely loosening the filter cakes from the filtercloths in the filter chambers during the cake discharge period. Anadditional purpose of the invention is to suggest a device to carry outsaid method.

The method according to the present invention advantageously can becombined with a vibration of all of the filter cloths at the same timeaccording to what is set forth in U.S. Pat. No. 4,900,454. The main partof the filter cakes can then be loosened and discharged according to theknown method by means of vibrations, while the method according to thepresent invention is used to loosen possibly remaining filter cakeresidues.

In a first embodiment of the invention the filter cakes are subjected toshearing forces, because the lower part of the filter cloths are movedlaterally in their plane direction. When the filter cloths are movedlaterally, folds are formed where the filter cakes or the filter cakeresidues are located and consequently substantial shearing forces areproduced in those parts of the filter cakes located closest to thefilter cloths and the filter cakes are loosened from the filter cloths.

According to a second embodiment of the invention the filter cakes aresubjected to sudden acceleration forces by lifting the rail guides whichsupport the filter cloths and letting them fall against stationarystops. When the falling filter cloths suddenly are braked particularlythe larger remaining residues of the filter cakes are loosened. Thus,this method is principally different from the already known method withvibrations to loosen the filter cakes. Large amounts of remaining filtercake residues have a pronounced suppressing influence on the amplitudeof the vibrations. A lower vibration amplitude of course lessens thecake loosening effect of the vibrations.

Also, the vibration amplitude can be different in different parts of thepressure filter. The vibration amplitude can be smaller particularly inthe central filter cells due to various damping effects. Such adifference in motion amplitude does not exist when the guide rails arelifted and then fall against stationary stops.

In accordance with another advantageous embodiment of the invention thesuspension means of the filter cloths are subjected to blows, preferablyin an upward direction, e.g. by means of a blow device and/or a drophammer, acting directly or indirectly. These blow devices produce aneffect, which resembles a lifting and a falling of the guide rails.

According to an additional advantageous embodiment of the invention apresence of possibly remaining filter cakes is detected by means of loadsensors, which transmit a signal to the process control unit. Theprocess control unit may be designed, in case remaining filter cakeresidues are present, to transmit a control signal to start one orseveral of the above-mentioned cake loosening processes and/or totransmit a warning signal and/or interrupt the filtration process infavor of a manual cleaning. If such a load sensor is used, one mayconfine oneself to using said above-mentioned cake loosening processesat those occasions when there really are remaining cake residuespresent. In this way the mechanical stresses on the suspension means ofthe filter cloths are reduced.

The invention will now be described in more detail by means of a fewembodiments, reference being made to the attached, partly schematicdrawings, in which:

FIG. 1 shows a pressure filter according to the invention, provided withblow devices;

FIG. 2 a schematic cross section of a pressure filter in a closedposition;

FIG. 3 a section which corresponds to FIG. 2 but showing an openposition and also showing remaining filter cake residues;

FIG. 4a and 4b a detail of an embodiment showing eccentric liftingdevices and stationary stop surfaces;

FIG. 5 a cross section of a filter according to the invention showingmeans for moving the lower part of the filter cloths;

FIG. 6 a detail of the above-mentioned moving means; and

FIG. 7 a cross section of a guide rail and a blow device fastened to theguide rail.

FIG. 1 shows a pressure filter 1, known per se. This pressure filtercorresponds to the pressure filter, which is described in U.S. Pat. No.4,900,454, reference expressly being made to this specification asregards its design and function, with the exception of those instanceswhich are directly connected with the present invention. The pressurefilter comprises mainly a frame 2 , which supports a fixed gable plate3, a movable gable plate 4 and pressure plates 5, 6 arranged betweensaid gable plates. The pressure plates can be pressed against each otherand brought apart from each other by means of hydraulic cylinders 7. Asshown in U.S. Pat. No. 4,900,454, the pressure plates have drainchannels for discharging the filtrate. Pressure plates 3, 4, 5, 6 formin pairs between each other pressure chambers 8, in each of which twofilter cloths 9 are placed.

The filter cloths are supported by suspension rods 10, which are movablein a horizontal direction on two flexibly or elasticly supported guiderails 12. The rails are at their two ends supported by flexible orelastic supporting elements 14, e.g. rubber blocks or helical springs.Elements 14 are carried by supports 16, which can comprise separateposts fixed to the floor or a portion of frame 2 or consoles projectingfrom the frame. Guide rails 12 are connected to each other at their endsby means of transversal yokes 18, which support vibrators 20. Thepressure filter is also provided with an inlet conduit A for materialand outlet conduits B, C, D, E for filtrate as well as an inlet conduitF for a pressure fluid.

FIG. 2 and 3 show the pressure plates and the filter cloths in detail.Hollow suspension rods 10 are connected to rinsing water inlets G andare provided with upwardly directed positioning ribs 22 and rinsingnozzles 24. The filter cloths are provided with a divisible inlet sleeve26 and rods 28 inserted in the lower hems. The rods on the respectivesides of each pressure plate 5,6 are connected at their ends by rubberbands 30. FIG. 3 illustrates also how residues of filter cake 32 stickto filter cloths 9 in various manners, when the pressure filter is in anopen position.

In order to loosen such residues of filter cakes 32, which could not beloosened solely by vibrations, sudden acceleration forces are producedby indirect impact by means of drop hammers 34, 36, which fall from anelevated position onto pivotally mounted arms 38. Arms 38 abut yokes 18of guide rails 12 with their opposite ends. However, it is also possibleto, in other ways, subject suspension means 10, 12 of filter cloths 9 todirect impact preferably upwardly directed blows by means of drophammers or other types of hammers.

FIG. 4 shows a detail of one end of the guide rails according to anotherembodiment of the invention, which includes eccentric lifting means 40and stop surfaces, onto which the guide rails can drop. In this casesupports 16 carry a cross bar 42 on top of elastic support elements 14,which cross bar is designed to carry the guide rails when they are in anot lifted position. A vibrator 20 (not shown in FIG. 4) suitably isfastened to the center of the lower side of the cross bar andconsequently is protected from blow stresses. Four guide rods 44 projectupwardly from cross bars 42 or supports 16 and run through guide sleeves46 fastened to the guide rails. Between support elements 14 and supports16 load sensors 43, e.g. pressure transmitters, are mounted, which areconnected to the process control unit (not shown) of the pressurefilter.

Eccentric lifting means 40 are mounted in pairs on shafts 50 anddesigned to abut the projecting ends of guide rails 12 or yokes 18 whichconnect them. They are mounted in an arbitrary way on supports 52 (notshown in detail), e.g. posts erected on the floor. Shafts 50 aredesigned to be rotated jointly with one or several driving means (notshown in detail), e.g. by means of chain driving or gear motors.Eccentric means 40 are provided with an elevated part 54, from whichmatching guide rail 12 or yoke part 18 is to drop onto a semi-elevatedpart 56, which functions as an anvil or stop surface. The lowest part ofeccentric lifting means 40 is designed to normally be directed upwardlyand then does not abut guide rails 12. The latter are carried by crossbars 42 during the filtration period and during the vibration. Whenfilter cakes are discharged the eccentric lifting means are rotated onerevolution, the guide rails being lifted and then dropping. This liftingcycle possibly can be repeated, in case load sensors 43 detect remainingresidues of filter cakes 32. In case there are no filter cake residuesfrom the start, the process control unit can transmit a signal that alifting is not to take place.

FIG. 4b shows an alternative embodiment, in which the ends of the guiderails or the tranversal yokes are designed to fall onto separate movableanvils 60, which are designed to be moved to a position below the endsof the guide rails or the yokes in a lifted position. This can e.g. beaccomplished by means of pull rods 62, the other end of which is mountedon eccentric discs 64 on shafts 50. Eccentric discs 64 preferably areplaced outside corresponding eccentric lifting means 40 in order not tointerfere with them.

FIG. 5 and 6 show a device designed to loosen residues of filter cakes32 by agitating the filter cloths and in this way produce shearingforces. This device comprises two longitudinal bars 66 and fingers 68,which project therefrom and are positioned just opposite every secondpair of rods 28 in the lower hem of filter cloths 9. Said fingers can bepushed towards the ends of the rods by means of e.g. hydraulic cylinders70 and thus every second cloth pair will be pushed to the right andevery second to the left in the figure. Additional control meanspossibly can be used to move the bars in a longitudinal direction inorder to move the lower part of the cloths in an opposite direction. Themeans which are used to control the ends of rods 28 can according to theinvention also be controlled in another way, e.g. manually. This can bedone when an alarm signal has been transmitted by load sensors 43 due toremaining filter cake residues in the filter. This device, designed tocontrol the ends of rods 28, advantageously can be combined with therest of the methods for filter cake loosening described above.

FIG. 7 shows a device according to the invention designed to producesudden acceleration forces. In this device guide rails 12 are belowprovided with a transversal reinforcement flange 72. A few blow devices74, so called "rappers", are attached to this flange and designed tojointly produce blows directed upwards against the guide rails. Suchblow devices are already known in screen filters and are used i.a. toloosen adhering material from arched screens and slanting, inoperativescreens and in this way to prevent a clogging of the screens.

The present invention is not limited to the illustrated embodiments, butit is possible to combine, in an arbitrary manner, variouscharacterizing features of the invention, as the latter is set forth inthe description and the attached drawings and as it is defined in thefollowing patent claims.

We claim:
 1. A method of using a pressure filter having pressure platesin pairs forming at least one pressure chamber in which twosubstantially planar vertical filter cloths are carried by suspensionrods movable in a horizontal direction on movably supported guide rails,and means to feed a suspension of solid particles in a liquid underpressure into the pressure chamber and force the liquid through thefilter cloths, means for discharging filtered liquid from said chamber,the solid particles remaining between the filter cloths forming a filtercake, including the steps of taking apart the pressure plates, andmechanically agitating the suspension means to loosen the filter cakesfrom the filter cloths including the additional step ofsubjecting thecloths to shearing forces by moving the lower part of the filter clothslaterally in the plane direction of the filter cloths.
 2. A methodaccording to claim 1, wherein said step of mechanically agitatingincludes the step of subjecting the suspension means of the filtercloths to blows.
 3. A method according to claim 2 wherein said blows areupwardly-directed.
 4. A method according to claim 1, including the stepof sensing the presence of filter cake residues remaining on the clothsby load sensors responsive to the weight of cloths.
 5. A methodaccording to claim 1, wherein all of the filter cloths are vibratedwhile moving the lower parts of the cloths during the step ofmechanically agitating the suspension means to loosen the filter cake.6. A pressure filter comprising a frame, pressure plates, means to pressthe plates against each other, means for separating from each other inpairs to form at least one pressure chamber capable of being closed andopened, and two substantially planar vertical filter cloths for formingfilter cakes between the cloths in said chamber, suspension meanssuspending said cloths, movable in a horizontal direction on guiderails, and vibrating means to subject the suspension means to amechanical agitation when the chamber is opened in order to loosen thefilter cakes from the filter cloths said pressure filter includingadditional means to subject the filter cloths and the filter cakes toshearing forces.
 7. A pressure filter according to claim 6, wherein eachsaid filter cloth includes a lower hem part, and a rod in each said hempart, said additional means comprise means for pushing the lower hemparts the lower parts of the filter cloths laterally respectively inopposite directions in the plane of the filter cloths.
 8. A pressurefilter according to claim 6 including further means to subject thefilter cloths and the filter cakes to sudden acceleration forces.
 9. Apressure filter according to claim 8, characterized in that said furthermeans comprise blow devices positioned to act on the suspension means toproduce sudden acceleration forces.
 10. A pressure filter according toclaim 9, wherein said blow devices comprise drop hammers actingindirectly on said suspension means.
 11. A pressure filter according toclaim 9, wherein said blow devices comprise stop surfaces and liftingmeans operable to lift the suspension means and the filter cloths and todrop the suspension means against the stop surfaces to produce saidsudden acceleration forces.
 12. A pressure filter according to claim 6,including load sensors detecting any filter cake residues remaining onsaid cloths.
 13. A method of using a pressure filter having pressureplates in pairs forming at least one pressure chamber in which twosubstantially vertical filter cloths are carried by suspension rodsmovable in a horizontal direction on movably supported guide rails andmeans to feed a suspension of solid particle sin a liquid under pressureinto the pressure chamber and force the liquid through the filtercloths, means for discharging filtered liquid from said chamber, thesolid particles remaining between the filter cloths forming a filtercake, including the steps of taking apart the pressure plates andmechanically agitating the suspension means to loosen the filter cakesfrom the filter cloths including the additional step ofsubjecting thecloths to sudden acceleration forces by lifting the guide rails whichcarry the filter cloths and dropping the rails against stationary stops.14. A pressure filter having a frame, pressure plates, means to pressthe plates against each other, means for separating the plates from eachother in pairs to form at least one pressure chamber capable of beingclosed and opened, and two substantially vertical filter cloths forforming filter cakes between the cloths in each chamber, said clothssuspended from suspension means movable in a horizontal direction onguide rails, and vibrating means to subject the suspension means to amechanical agitation when the chamber is opened in order to loosen thefilter cakes from the filter cloths, said pressure filter includingfurther means to subject the filter cloths and the filter cakes tosudden acceleration forces.